In this presentation we discuss cobalt crusts, its classification, Occurrence and Distribution, Formation, Texture, Mineralogy, Scope for future mining and exploration.
2. On the slopes of submarine mountains around
the world, minerals precipitate out of the
seawater to form thin crusts on rocky surfaces.
The crusts are commonly called
ferromanganese crusts, reflecting the fact that
their major constituents are iron (Fe) and
manganese (Mn), although a host of other
minerals occur in them in smaller amounts,
including cobalt - which is why they are also
often called ‘cobalt-rich crusts’ or ‘cobalt-rich
ferromanganese crusts’.
3. There are two primary practical interest in Fe-
Mn crusts
Their economic potential for Co, but also for
Mn, Ni, and Pt and possibly also Ti, REE, Te,
P, and others
Use of crusts as recorders of the past 60 Ma
of oceanic and climatic history.
4. Up untill 1970s Fe-Mn crusts were usually not
distinguished from Fe-Mn Nodules however there are
distinct differences between Fe-Mn nodules and
crusts, other than just form.
Nodules commonly form by both diagenetic and
hydrogenetic processes and thus their composition
reflects input from both sea water and sediment pore
water sources, the latter being substrate contribution,
which is not found in crusts.
Generally, crusts and nodules have different
mineralogical (vernadite vs. todorokite and vernadite)
and chemical compositions( eg: High Co vs. High Cu)
because of their genetic differences as well as
differences in water depth of formation, although
there is much overlap
5. Cobalt-rich ferromanganes crust are found throughout the global oceans
on the flanks and summits of seamounts (submarine mountains), ridges
and plateaus, where seafloor currents have swept the ocean floor clear
of sediment for millions of years. They form at water depths of 600m to
7000m. Crusts of sufficient mineral content to be of economic interest
commonly occur at depths of about 800m to 2500m. Their thickness
varries from 1mm to about 260mm. These seamounts can be huge,
some as large as mountain ranges on the continents. Only a few of the
estimated 30,000 seamounts that occur in the Pacific, where the richest
deposits are found, have been mapped and sampled in detail. The
Atlantic and Indian oceans contain far fewer seamounts but have been
far less sampled.
Based on grade, tonnage and oceanographic conditions, the central
equatorial Pacific region offers the best potential for crust mining,
particularly the exclusive economic zones around Johnston Island and
Hawaii (United States), the Marshall Islands, the Federated States of
Micronesia and international waters of the mid-Pacific.
10. Formation of cobalt-rich ferromanganese crusts.
Adapted and modified from Hein 2004.
O2 Minimum zone
1)Oxidation of Mn2+ and formation of
colloids
2) Precipitation of Oxides/
oxyhydroxides
11. Even though Fe-Mn crusts form by hydrogenetic precipitation, the exact mechanism
of metal enrichment in the water column and at crust surface are poorly understood.
The ultimate sources of metals to the oceans are river and aeolian input,
hydrothermal input, weathering of basalts, release of metals from sediments, and
extraterrestrial input. Scientific models show that most hydrogenetic elements in
crusts occur as inorganic complexes in sea water. Hydrated cations
(Co,Ni,Zn,Pb,Cd,Tl, etc.) are attracted to the negatively charged surface of Mn
oxyhydroxides, whereas anions and elements that form large complexes with low
charge-density (V,As,P,Zr,Hf, etc.) are attracted to the slightely positive charge of
Fe Hydroxide surfaces.
Mixed Fe and Mn colloids with adsorbed metals precipitate onto hard-rock surfaces as
poorly crystalline or amorphous oxyhydroxides, probably through bacterially
mediated catalytic processes.
The metals are adsorbed because of the crusts’ very slow growth rates (1 to 5
millimetres per million years) and the enormous specific surface area(average 325
square metres per cubic centimetre of crust) (Hein et al. 2000)
12. Crust surface directly exposed on the seafloor are botryoidal, with
botryoids varying in size from microbotryoidal (millimeter size)
to botryoidal (centimeter size).
Crust profiles vary according to thickness and regional
oceanographic conditions. Thin crusts(<40 mm) are usually
black and massive, botryoidal or laminated. Thicker crusts (40-
80 mm) commonly have atleast two distinct layers, a lower black,
massive, dense layer that is phosphatized and an upper black to
brown layer that is more porous, with laminated, mottled,
botryoidal, and/or columnar textures.
The various textures probably reflect bottom water conditions at
the time of precipitation of the oxides. Mottled, columnar,
botryoidal, and laminated textures probably represent
progressively decreasing energy in the depositional environment.
13.
14. Ferromanganese crusts have a simple mineralogy. They are
composed predominantly of vernadite (manganese oxide, or
MnO2) and non-crystalline iron oxyhydroxide (FeOOH).
Ferromanganese crusts also contain minor amounts of detrital
minerals, such as quartz and feldspar and authigenic carbonate
fluorapatite.
Cobalt, the trace metal of greatest economic interest, can be up to
2 percent, but usually averages 0.5 to 0.8 per cent by weight.
Ferromanganese crusts contain the highest concentrations of the
rare metal tellurium, which is used in the solar cell industry to
produce thin-film photovoltaics – the best material for
converting sunlight into electricity.
Other important metalic and rare earth elements found in crusts
include titanium, cerium, nickel, platinum, phosphorus, thallium,
zirconium, , tungsten, bismuth and molybdenum.
15.
16. To locate areas likely to be productive, prospective miners will first have to
develop detailed maps of crust deposits and a comprehensive, small-scale
picture of seamount topography, including seismic profiles. Once sampling
sites are identified, dredge hauls, core samplers, sonar and video cameras
can be deployed to ascertain crust, rock and sediment types and
distribution. Large, well equipped research vessels will be needed to
operate bottom acoustic beacons and towed equipment, and to handle a
large number of samples. Manned submersibles or remotely operated
vehicles will be required in later stages. For environmental assessment,
current-meter moorings and biological sampling equipment will have to be
deployed.
For successful crust mining, it is essential to recover the crusts without
collecting too much substrate, which would substantially dilute the ore
quality. One possible method of crust recovery consists of a bottom-
crawling vehicle attached to a surface vessel by a hydraulic-pipe lift system
and an electrical umbilical Some innovative systems that have been
suggested include water-jet stripping of crusts from the rock, chemical
leaching of the crusts while they are still on the seamounts
17. Cronan, handbook of marine mineral
deposits,2000, CRC press, london. pp 239
E. Baker and Y. Beaudin, Secretariat of the
Pacific Community (2103) Deep Sea Minerals:
CobaltrichFerromanganese Crusts, a physical,
biological, environmental, and technical
review. Vol. 1C, SPC
International seabed authority, Cobalt bearing
ferromanganese crustssampling points in the
world's oceans v2-Mar08
